68637480 comments ability to edit and delete

https://www.pivotaltracker.com/story/show/68637480

- added base edit and update functionality
- added comment permissions policy via pundit
- added Rspec checks on edit and delete comment
- added Rspec checks for permission rules:
  - check on comment owner
  - check on time passed (comment can be updated or deleted only in 5 minutes after posting)